David Durk was a police academy student in the early 1960s, he says, when an instructor advised, during a formal lecture, that he carry a self-addressed stamped envelope at all times. This way, if offered a bribe, he could drop the money in a mailbox without fear that it would be discovered later. As the public would eventually learn, the New York City police force that Durk joined was rife with corruption at every level. “Being a cop, you see a whole different part of New York City,” he says. “It’s real and scary.”

Durk and Detective Frank Serpico broke the story of widespread graft to The New York Times, which reported in April 1970 that “narcotics dealers, gamblers and businessmen make illicit payments of millions of dollars a year to the policemen of New York.” The efforts of Durk and Serpico, dramatized in the 1973 film Serpico, led New York City Mayor John Lindsay to create the Knapp Commission, which exposed extortion operations being run out of most precinct houses. Amherst magazine published Durk’s testimony to the commission in 1970. “That was the price of going along, the real price of police corruption,” he told the commission: “broken dreams and dying neighborhoods and a whole generation of children being lost. That was what I had joined the department to stop.”

Serpico left the force in 1972 after sustaining a critical gunshot wound, but Durk stuck with it, and though “transferred umpteen times,” he says, he continued to ferret out institutional vice. As commander of detectives in East Harlem, he challenged orders from a superior not to make arrests—even for robberies with guns, he says. As assistant commissioner for tax enforcement under Mayor Ed Koch, Durk doggedly pursued tax evasion cases, recovering millions of dollars. He contends that a variety of supervisors and political figures discouraged him from reporting corruption at the highest levels of corporate and political life in the city. Told by the department that he “would not be welcomed back” for additional police assignments, he says, he retired in 1985 with a yearly pension of around $17,000. “I was defrocked and defenestrated,” he says.

Durk believes that a better-educated police force would think more critically about its role in society. Over the years, he’s lectured at Harvard, served as an adjunct instructor at Yale, taught night courses to New York City civil servants and consulted with several police departments. He says he’s been sought out many times by police officers who want to expose corruption on the force. But when Durk tells them his story, they think twice. “They say, ‘Why should I do this? Look what happened to you.’ I don’t have a rebuttal. That part is very sad.”

Perhaps surprisingly, Durk has fond memories of his time on the force. “I loved being a cop—it’s like your last chance to be a knight-errant in our society,” he says. And, for all he’s seen, he still believes that police work can be a powerful way to serve humanity and actualize equality and justice. Forty years later, he stands behind his testimony to the Knapp Commission: “Being a cop is a vocation or it is nothing at all.”